Message generation

Like all ROS Client Libraries, rospy takes msg files and generates Python source code for them. The pattern for this is:

• package_name/msg/Foo.msg → package_name.msg.Foo

Similarly, srv files also have Python source code generated. The pattern for this is:

• package_name/srv/Bar.srv → package_name.srv.Bar

The source for the generated files are in the src directory of package_name.

Thus, to use the std_msgs/String message in your code you would use one of the following import statements:

import std_msgs.msg
msg = std_msgs.msg.String()

or

from std_msgs.msg import String
msg = String()

Message initialization

There are three ways to initialize a new Message instance: in-order arguments (*args), keyword arguments (**kwds), no arguments.

No arguments

• In the no-arguments style you instantiate an empty Message and populate the fields you wish to initialize. For example

  msg = std_msgs.msg.String()
  msg.data = "hello world"

  Message fields are given default values, which means you can do direct field assignment into embedded messages like a Header:

  msg = sensor_msgs.msg.Imu()
  msg.header.stamp = rospy.Time.now()

In-order arguments (*args):

• In the in-order style, a new Message instance will be created with the arguments provided, in order. The argument order is the same as the order of the fields in the Message, and you must provide a value for all of the fields. For example, std_msgs.msg.String only has a single string field, so you can call:

  msg = std_msgs.msg.String("hello world")

  std_msgs.msg.ColorRGBA has four fields (r, g, b, a), so we could call:

  msg = std_msgs.msg.ColorRGBA(255.0, 255.0, 255.0, 128.0)

Keyword arguments (**kwds)

• In the keyword style, you only initialize the fields that you wish to provide values for. The rest receive default values (e.g. 0, empty string, etc...). For std_msgs.msg.String, the name of its lone field is data, so you can call:

  msg = std_msgs.msg.String(data="hello world")

  std_msgs.msg.ColorRGBA has four fields (r, g, b, a), so we could call:

  msg = std_msgs.msg.ColorRGBA(b=255)

  which would publish a ColorRGBA instance with b=255 and the rest of the fields set to 0.0.

Each style has benefits and trade-offs. The keywords style is the recommended approach. It is resilient to many types of msg changes (new fields, field re-ordering) and is often more concise than other approaches. The in-order style is beneficial when you want your code to be brittle to msg changes, such as with regressions tests or other code that you wish to notice message changes. The no arguments style requires more lines of code but is useful if your message has embedded Message instances: after instantiation, these embedded messages will have been instantiated and you can do direct field assignments into them (e.g. setting the timestamp of a Header).